Magnetic marker or tag

ABSTRACT

Magnetic tags or markers are disclosed, together with a variety of techniques by means of which such tags may be interrogated. In one aspect, the magnetic marker or tag which is characterised by carrying a plurality of discrete magnetically active regions in a linear array. In another aspect, the invention provides a method of interrogating a magnetic tag or marker within a predetermined interrogation zone, the tag compromising a high permeability magnetic material, for example to read data stored magnetically in the tag or to use the response of the tag to detect its presence and/or to determine its position within the interrogation zone, characterized in that the interrogation process includes the step of subjecting the tag sequentially to: (1) a magnetic field sufficient in field strength to saturate the high permeability magnetic material, and (2) a magnetic null as herein defined. Applications of such techniques are described, inter alia, in relation to (a) identifying articled to which tags are attached; (b) accurate determination of position, as in the location of surgical probes; (c) totalisation of purchases, where each item carries a tag coded with data representing its nature and its price.

RELATED PATENT APPLICATION

This is a division of Ser. No. 08/930,228, filed Jan. 12, 1998, now U.S.Pat. No. 6,144,300, issued Nov. 7, 2000.

BACKGROUND OF THE INVENTION

This invention relates to the exploitation of magnetic properties in arange of practical techniques, and utilizes a new technique of spatialmagnetic interrogation in conjunction with a magnetic marker oridentification tag. More particularly, but not exclusively, theinvention relates to methods of determining the presence and/or thelocation of a magnetic marker or tag within an interrogation zone; tomethods of identifying a magnetic tag (e.g. identifying a given tag inorder to discriminate that tag from others); to systems for puttingthese methods into practice; to magnetic tags for use in such methodsand systems; and to the storage of data in such tags, and the subsequentremote retrieval of data from such tags.

It should be understood that the terms “tag” and “marker” are usedherein interchangeably; such devices may be used in many differentapplications and, depending on the magnetic qualities of the device, mayserve to denote (a) the mere presence of the tag (and hence that of anarticle to which the tag is attached); or (b) the identity of the tag(and hence that of an article to which it is attached); or they mayserve to define the precise position of the tag with respect topredetermined coordinates (and hence that of an article to which it isattached); or they may serve to provide access codes (e.g. for entryinto secure premises; or for ticketing purposes, e.g. on publictransport networks); or they may serve generally to discriminate onearticle or set of articles from other articles.

In addition, the terms “AC field” and “DC field” are used herein todenote magnetic fields whose characteristics are, respectively, thoseassociated with an electrical conductor carrying an alternating current(AC) or a direct current (DC).

The tags, methods and systems of this invention have a wide variety ofapplications as indicated above. These include (but are not restrictedto) inventory control, ticketing, automated shopping systems, monitoringwork-in-progress, security tagging, access control, anti-counterfeiting,and location of objects (in particular the precise positioning ofworkpieces (e.g. probes in surgery)).

PRIOR ART

There are a number of passive data tag systems currently available. Themost widely-used is based on optically-read printed patterns of lines,popularly known as barcodes. The tag element of such systems is verylow-cost, being typically just ink and paper. The readers are alsorelatively low cost, typically employing scanning laser beams. For manymajor applications the only real drawback to barcodes is the need forline-of-sight between the reader and the tag.

For applications where line-of-sight is not possible, systems notemploying optical transmission have been developed. The most popularemploy magnetic induction for coupling between the tag and theinterrogator electronics. These typically operate with alternatingmagnetic fields in the frequency range of 50 kHz to 1 MHz, and generallyemploy integrated electronic circuits (“chips”) to handle receive andtransmit functions, and to provide data storage and manipulation. Inorder to avoid the need for a battery, power for the chip is obtained byrectification of the interrogating signal received by an antenna coil.In order to increase the power transferred, and to providediscrimination against unwanted signals and interference, the coil isusually resonated with a capacitor at the frequency of the interrogationsignal carrier frequency. A typical product of this type is the TIRISsystem manufactured by Texas Instruments Ltd.

Other multi-bit data tag systems have employed conventional h.f. radiotechnology, or technologies based on surface acoustic waves ormagnetostriction phenomena.

FIELD OF THE INVENTION

The present invention involves, inter alia, the use of a new type ofpassive data tag system which employs small amounts of veryhigh-permeability magnetic material, and a scanned magnetic field forinterrogation. Since the magnetic material can be in the form of a thinfoil, wire or film, it can be bonded directly to a substrate, e.g. paperor a plastics material, to form self-supporting tags.

Alternatively, the magnetic material may be incorporated into thestructure of an article with which the tag is to be associated; thus atag may be formed in situ with the article in question by applying themagnetic material to the surface of the article, or by embedding themagnetic material within the body of the article.

The invention exploits magnetic fields which contain a “magneticnull”—this term is used herein to mean a point, line, plane or volume inspace at or within which the component of the magnetic field in a givenlinear direction is zero. The volume in space over which this conditionis met can be very small—and this gives rise to certain embodiments ofthe invention in which precise position is determined. Typically themagnetic null will be extant over a relatively small linear range. Itshould be understood that, where there is a magnetic null, it ispossible (and is often the case) that the magnetic field component in adirection orthogonal to the given linear direction will be substantial.In some embodiments of this invention, such a substantial orthogonalfield is desirable.

One way of creating the magnetic null is to employ opposing magneticfield sources. These may be current-carrying coils of wire, or permanentmagnets (these being well suited to small-scale systems), orcombinations of coil(s) and permanent magnet(s). It is also possible toexploit the magnetic nulls which exist in specific directions when asingle coil or permanent magnet is used.

For large scale applications, the magnetic field sources are preferablycoils carrying direct current.

The invention also utilizes the relative movement between a magneticmarker and an applied magnetic field in order to effect passage over themarker of the magnetic null. This can be achieved by moving the markerwith respect to the applied magnetic field, or by holding the marker ina fixed position while the magnetic field is scanned over it. Generally,the invention exploits the difference between the magnetic behavior ofthe marker in (i) a zero field (at the magnetic null), and (ii) in ahigh, generally saturating, magnetic field.

Tags of this Invention

According to one aspect of the present invention, there is provided amagnetic marker or tag which is characterized by carrying a plurality ofdiscrete magnetically active regions in a linear array. The discretemagnetically active regions may be supported on a substrate, e.g. paperor a plastics material, or they may be self-supporting. Alternatively,the magnetic elements may be incorporated directly into or onto articlesduring manufacture of the articles themselves. This is appropriate, forexample, when the articles are goods, e.g. retail goods, which carry thetags for inventory purposes; or when the articles are tickets orsecurity passes.

A tag as defined above can also be formed from a continuous strip ofhigh permeability material, discrete regions of which have theirmagnetic properties permanently or temporarily modified. It will beappreciated that such a process can begin with a high permeability stripselected regions of which are then treated so as to modify theirmagnetic properties, generally by removing or reducing their magneticpermeability; or with a strip of high permeability magnetic materialaccompanied by a magnetizable strip positioned close to the highpermeability magnetic material, e.g. overlying it or adjacent to it,selected regions of which are magnetized. In relatively simpleembodiments, each magnetically active region has the same magneticcharacteristics; in more complex embodiments, each magnetically activeregion can possess a different magnetic characteristic, thus making itpossible to assemble a large number of tags, each with unique magneticproperties and hence with a unique magnetic identity and signature (whenprocessed by a suitable reader device).

Because the invention utilizes relative movement between a tag and anapplied magnetic field, it will be appreciated that there will be acorrespondence between the time domain of output signals from a tagreading device and the linear dimensions of the magnetically activeregions of a tag and of the gaps between the magnetically activeregions. In this sense, the active regions and the gaps between themfunction analogously to the elements of an optical bar code (black baror white gap between adjacent bars). It follows from this that, just asvariability of magnetic characteristics in the active regions can beused to generate part of a tag “identity”, so can the linear spacingbetween adjacent magnetically active regions. It will readily beunderstood that a vast number of tags, each with its own uniqueidentity, can thus be produced in accordance with this invention.

Although the tags have been described as possessing a linear array ofmagnetically active regions, the tags may in fact have two or more suchlinear arrays. These may be disposed mutually parallel, or mutuallyorthogonal, or in any desired geometrical arrangement. For simplicity ofreading such tags, arrays which are parallel and/or orthogonal arepreferred.

Appropriate techniques for manufacturing the tags of this invention arewell-known in conventional label (i.e. magnetic marker) manufacture.Suitable magnetic materials are also well-known and widely available;they are high-permeability materials which preferably have an extrinsicrelative permeability of at least 10³. The coercivity of the magneticmaterial will depend on the tag's intended use. The magnetic material ispreferably in the form of a long thin strip or of a thin film; theseformats avoid major internal demagnetization effects. Suitable stripmaterials are readily available from commercial suppliers such asVacuumschmeltze (Germany), Allied Signal Corp. (USA), and Unitika(Japan). Thin film material currently manufactured in high volume by IST(Belgium) for retail security tag applications is also suitable for usein this invention.

Detection/Identification Methods

As well as the tags defined above, the present invention provides avariety of useful methods for detecting the presence of a magneticmarker and/or for identifying such a marker. While in many cases thesemethods will be intended for use in conjunction with the tags of theinvention, this is not a necessary prerequisite in the methods of theinvention.

According to a second aspect of the invention, there is provided amethod of interrogating a magnetic tag or marker within a predeterminedinterrogation zone, the tag comprising a high permeability magneticmaterial, for example to read data stored magnetically in the tag or touse the response of the tag to detect its presence and/or to determineits position within the interrogation zone, characterized in that theinterrogation process includes the step of subjecting the tagsequentially to: (1) a magnetic field sufficient in field strength tosaturate the high permeability magnetic material, and (2) a magneticnull as herein defined.

Preferably the magnetic null is caused to sweep back and forth over apredetermined region within the interrogation zone. The scanningfrequency (i.e. the sweep frequency of the magnetic null) is preferablyrelatively low, e.g. 1-500 Hz. Conveniently, the field pattern isarranged so that (a) said magnetic null lies in a plane; and (b) thesaturating field occurs adjacent to said plane.

According to a third aspect of this invention, there is provided amethod of determining the presence and/or the position of a magneticelement within a predetermined interrogation zone, the magnetic elementhaving predetermined magnetic characteristics, which method ischaracterized by the steps of: (1) establishing within saidinterrogation zone a magnetic field pattern which comprises a relativelysmall region of zero magnetic field (a magnetic null) contiguous withregions where there is a magnetic field sufficient to saturate the, or apart of the, magnetic element (the saturating field), said relativelysmall region being coincident with a region through which the magneticelement is passing, or can pass, or is expected to pass; (2) causingrelative movement between said magnetic field and said magnetic elementsuch that said magnetic null is caused to traverse at least a part ofthe magnetic element in a predetermined manner; and (3) detecting theresultant magnetic response of the magnetic element during said relativemovement.

According to a fourth aspect of the present invention, there is provideda method of identifying a magnetic element which possesses predeterminedmagnetic characteristics, which method is characterized by the steps of:(1) subjecting the magnetic element to a first magnetic field which issufficient to induce magnetic saturation in at least a part of themagnetic element; (2) next subjecting the magnetic element to conditionsof zero magnetic field (i.e. a magnetic null), the zero field occupyinga relatively small volume and being contiguous with said first magneticfield; (3) causing relative movement between the applied magnetic fieldand said magnetic element such that said magnetic null is caused totraverse at least a part of the magnetic element in a predeterminedmanner; and (4) detecting the resultant magnetic response of themagnetic element during said relative movement.

In the identification method defined above, the magnetic element isadvantageously caused to traverse an interrogation zone within which therequired magnetic conditions are generated.

In a fifth aspect, the invention provides a method of identifying amagnetic element, the magnetic element having predetermined magneticcharacteristics, which method is characterized by the steps of: (1)causing the magnetic element to enter an interrogation zone within whichthere is established a magnetic field pattern which comprises arelatively small region of zero magnetic field (a magnetic null)contiguous with regions where there is a magnetic field sufficient tosaturate the, or a part of the, magnetic element (the saturating field);(2) causing the magnetic element to be moved through the saturatingfield until it reaches the magnetic null; (3) causing relative movementbetween said magnetic field and said magnetic element such that saidmagnetic null is caused to traverse at least a part of the magneticelement in a predetermined manner; and (4) detecting the resultantmagnetic response of the magnetic element during said relative movement.

The relative movement between the magnetic element and the magneticfield may advantageously be produced by sweeping the applied magneticfield over the magnetic element. Alternatively, the relative movementcan be achieved by the application of an alternating magnetic field to agenerally static magnetic field pattern.

In carrying out the methods defined above, preferred embodiments of themagnetic element are either elongate, and the magnetic null is thenarranged to extend along the major axis of said magnetic element; orthey are in the form of a thin film, in which case the magnetic null isarranged to extend to be aligned with the axis of magnetic sensitivityof the thin film material.

The magnetic field or field pattern utilized in the methods definedabove may be established by the means of two magnetic fields of oppositepolarity. This can conveniently be achieved by use of one or more coilscarrying direct current; or by the use of one or more permanent magnets;or by a combination of coil(s) and magnet(s).

Where a coil is used, it may be arranged to carry a substantiallyconstant current so as to maintain the magnetic null at a fixed point.Alternatively, the coil(s) carry/carries a current whose magnitudevaries in a predetermined cycle so that the position of the magneticnull is caused to oscillate in a predetermined manner. We describe thisas a “flying null’. A similar arrangement can be used to give a flyingnull when both a coil or coils and a permanent magnet are used.

According to a further aspect of the present invention, there isprovided a method of determining the presence and/or the position of amagnetic element, which is characterized by the steps of: (1) applying amagnetic field to a region where the magnetic element is, or is expectedto be, located, said magnetic field comprising two opposed fieldcomponents, generated by magnetic field sources, which result in a nullfield (a magnetic null) at a position intermediate said magnetic fieldsources (which position is known or can be calculated); (2) causingrelative movement between said magnetic field and said magnetic element;and (3) detecting the resultant magnetic response of the magneticelement during said relative movement.

Relative movement between the magnetic field and the magnetic elementmay be achieved by applying a relatively low amplitude alternatingmagnetic field superimposed on the DC field. Typically, such a lowamplitude alternating magnetic field has a frequency in the range from10 Hz to 100 kHz, preferably from 50 Hz to 50 kHz, and mostadvantageously from 500 Hz to 5 kHz.

In one embodiment, the coils carry a substantially constant current soas to maintain the magnetic null at a fixed point. In anotherembodiment, the coils carry a current whose amplitude varies in apredetermined cycle so that the position of the magnetic null is causedto oscillate in a predetermined manner.

In the methods according to this invention, detection of the magneticresponse of the magnetic element advantageously comprises observation ofharmonics of the applied AC field which are generated by the magneticelement as its magnetization state is altered by passing through themagnetic null.

As indicated above, the system operates with a zero or very lowfrequency scanning field, and an HF (high frequency) in the range 50Hz-50 kHz. This allows for good signal penetration through mostmaterials including thin metal foils. In addition, internationalregulations allow high fields for transmission at these low frequencies.

Preferred embodiments of the invention provide a multi-bit data tagsystem which employs low-frequency inductive magnetic interrogation, andavoids the need for complex, expensive tags.

According to another aspect of the present invention, there is provideda method of coding and/or labeling individual articles within apredetermined set of articles by means of data characteristic of thearticles, e.g. article price and/or the nature of the goods constitutingthe articles, which method is characterized by applying to each articlea magnetic tag or marker carrying a predetermined arrangement ofmagnetic zones unique to that article or to that article and otherssharing the same characteristic, e.g. article price or the nature of thegoods constituting the article, said magnetic tag or marker beingsusceptible to interrogation by an applied magnetic field to generate aresponse indicative of the magnetic properties of the tag or marker andhence indicative of the nature of the article carrying the magnetic tagor marker.

Fundamentals of the Invention

Before describing further embodiments, it will be helpful to explainsome fundamental aspects of the invention, giving reference whereappropriate to relatively simple embodiments.

A key aspect of the invention is the form of the magnetic field createdin the interrogation zone; as will become apparent later, this fieldallows very small spatial regions to be interrogated. The means forgenerating this magnetic field will be termed hereinafter an“interrogator”. In one simple form, the interrogator consists of a pairof closely-spaced identical coils arranged with their axes coincident.The coils are connected together such that their winding directions areopposed in sense, and a DC current is passed through them. This causesopposing magnetic fields to be set up on the coils' axis, such that aposition of zero field—a magnetic null—is created along the coil axis,mid-way between the coils. The level of current in the coils is such asto heavily saturate a small sample of high permeability magneticmaterial placed at the center of either of the two coils. A much loweramplitude AC current is also caused to flow in opposite directionsthrough the two coils, so that the AC fields produced sum togethermidway between the coils. This can easily be arranged by connecting asuitable current source to the junction of the two coils, with a groundreturn. The frequency of this AC current may typically be about 2 kHz,but its value is not critical, and suitable frequencies extend over awide range. This AC current generates the interrogating field whichinteracts with a magnetic tag to generate a detectable response. Anothereffect of this AC current is to cause the position of zero field—themagnetic null—to oscillate about the mid-way position along the coils'axis by a small amount (this is a wobble or oscillation rather than anexcursion of any significant extent).

In addition, a further, low frequency AC current may be fed to the coilsso as to generate a low frequency scanning field (which may be zero).The frequency of the scanning field (when present) should besufficiently low to allow many cycles of the relatively high frequencyinterrogation field to occur in the time that the magnetic null regionpasses over the tag; typically, the frequency ratio of interrogatingfield (ω_(c)) to the scanning field (ω^(b)) is of the order of 100:1,although it will be appreciated that this ratio can vary over aconsiderable range without there being any deleterious effect on theperformance of the invention.

When a tag containing a piece of high-permeability magnetic material ispassed along the coils' axis through the region over which oscillationof the magnetic zero plane occurs, it will initially be completelysaturated by the DC magnetic field. It will next briefly be driven overits B-H loop as it passes through the zero field region. Finally it willbecome saturated again. The region over which the magnetic material is“active”, i.e. is undergoing magnetic changes, will be physically small,and is determined by the amplitude of the DC field, the amplitude of theAC field, and the characteristics of the magnetic material. This regioncan easily be less than 1 mm in extent. If the level of the alternatingfield is well below that required to saturate the magnetic material inthe tag, then harmonics of the AC signal will be generated by the tag asit enters the zero field region of interrogator field and responds tothe changing field. As the tag straddles the narrow zero field regionthe tag will be driven on the linear part of its B-H loop, and willinteract by re-radiating only the fundamental interrogation frequency.Then, as the tag leaves the zero field region, it will again emitharmonics of the interrogation field frequency. A receiver coil arrangedto be sensitive to fields produced at the zero field region, but whichdoes not couple directly to the interrogator coils, will receive onlythese signals. The variation of these signals with time as the tagpasses along the coils axis gives a clear indication of the passage ofthe ends of the magnetic material through the zero field region.

It will be appreciated that because the interrogation zone can be verynarrow, each individual piece of magnetic material can be distinguishedfrom its neighbors, from which it is separated by a small distance.Naturally, the magnetic material will be selected to suit the particularapplication for which the tag is intended. Suitable magnetic materialsare commercially available, as described hereinbefore.

If a tag containing a number of zones or pieces of magnetic materialplaced along the axis of the label is now considered, it will beappreciated that as each zone or piece of magnetic material passesthrough the zero-field region, its presence and the positions of itsends can be detected. It then becomes a simple matter to use the lengthsand spacing of individual zones or pieces of magnetic material torepresent particular code sequences. Many different coding schemes arepossible: one efficient arrangement is to use an analogue of the codingscheme used for optical barcodes, where data is represented by thespacing and widths of the lines in the code.

The system so far described allows for the scanning of a single-axis tag(e.g. a wire or a thin strip of anisotropic material, having a magneticaxis along its length) as it physically moves through the coil assembly.It will be appreciated that relative movement between the tag and theinterrogating field can be achieved either with the field stationary andthe tag moving, or vice versa. If required, the arrangement can be madeself-scanning, and thus able to interrogate a stationary tag, e.g. bymodulating the d.c. drive currents to the two interrogator coils, sothat the zero field region scans over an appropriate portion of the axisof the coils. The extent of this oscillation needs to be a: least equalto the maximum dimension of a tag, and should preferably be considerablygreater, to avoid the need for precise tag positioning within theinterrogation zone.

By using extra coils arranged on the 2 axes orthogonal to the original,tags in random orientations can be read by sequentially field scanning.This involves much greater complexity in the correlation of signals fromthe three planes, but because of the very high spatial resolutionavailable would be capable of reading many tags simultaneously presentin a common interrogation volume. This is of enormous benefit forapplications such as tagging everyday retail shopping items, and, forexample, would allow automated price totalization of a bag of shoppingat the point of sale. Thus the invention has applicability to the pricelabeling of articles and to point-of-sale systems which generate a salestotal (with or without accompanying inventory-related data processing)

The size of a simple linear tag is dependent on the length of theindividual elements, their spacing and the number of data bits required.Using strips of the highest permeability material commerciallyavailable, such as the “spin-melt” alloy foils available from supplierssuch as Vacuumschmeltze (Germany) and Allied Signal (USA), the minimumlength of individual elements which can be used is probably of the orderof a few millimeters. This is because the extrinsic permeability will bedominated by shape factors rather than by the very high intrinsicpermeability (typically 10⁵), and shorter lengths may have insufficientpermeability for satisfactory operation.

For this reason it is attractive to use very thin films of highpermeability magnetic material. Provided it is very thin, (ideally lessthan 1 μm), such material can be cut into small 2 dimensional pieces(squares, discs, etc.) with areas of just 20 mm² or less, yet stillretain high permeability. This will enable shorter tags than possiblewith elements made from commercially available high-permeability foils.Suitable thin film materials are available commercially from IST(Belgium).

An extension to this type of programming can also be used to prevent thecomposite tag producing an alarm in a retail security system (such analarm would be a false indication of theft, and would thus be anembarrassment both to the retailer and to the purchaser). If differentregions of the tag are biased with different static field levels, theywill produce signals at different times when they pass through retailsecurity systems. This will complicate the label signature in suchsystems and prevent an alarm being caused. In the present invention, thereading system will be able to handle the time-shifted signals caused bysuch magnetic biasing.

Thus far tag coding has been described on the basis of physicallyseparated magnetic elements. It is not essential, however, to physicallyseparate the elements; programming of data onto a tag may beaccomplished by destroying the high-permeability properties of acontinuous magnetic element in selected regions thereof. This can bedone, for example, by local heating to above the re-crystallizationtemperature of the amorphous alloy, or by stamping or otherwise workingthe material. Of even more importance is the ability to magneticallyisolate regions of a continuous element of high permeability material bymeans of a magnetic pattern stored on an adjacent bias element made frommedium or high coercivity magnetic material. Such a composite tag couldthen be simply coded by writing a magnetic pattern onto the bias elementusing a suitable magnetic recording head. If required, the tag couldthen be erased (by de-gaussing with an AC field) and re-programmed withnew data.

The scheme described can also be extended to operate with tags storingdata in two dimensions. This allows for much more compact tags, since aswell as being a more convenient form, a tag made up from an N×N array ofthin-film patches has much more coding potential than a linear array ofthe same number of patches. This is because there are many more uniquepatch inter-relationships that can be set up in a given area.

FURTHER EMBODIMENTS Use of Spatial Magnetic Scanning for PositionSensing

In addition to interrogating space to read data tags, this new techniqueof moving planes of zero field through space (or moving things throughthe planes) can be used to provide accurate location information forsmall items of high permeability magnetic material.

Thus, according to another aspect, the invention provides a method ofdetermining the precise location of an object, characterized in that themethod comprises: (a) securing to the object a small piece of a magneticmaterial which is of high magnetic permeability; (b) applying to theregion in which said object is located a magnetic field comprising twoopposed field components, generated by magnetic field sources, whichresult in a null field at a position intermediate said magnetic fieldsources; (c) applying a low amplitude, high frequency interrogatingfield to said region; (d) causing the position of the null field tosweep slowly back and forth over a predetermined range of movement; (e)observing the magnetic interaction between said applied magnetic fieldand said small piece of magnetic material; and (f) calculating theposition of the object from a consideration of said magnetic interactionand from the known magnetic parameters relating to said applied fieldand to said small piece of magnetic material. Advantageously, the smallpiece of high permeability magnetic material is in the form of a thinfoil, a wire or a thin film.

This aspect of the invention is of particular interest when the objectwhose location is to be determined is a surgical instrument, for examplea surgical probe or needle. The invention allows precise determinationof the location of, for example, a surgical probe during an operation.

This technique is ideal for accurate location of very small markerswithin relatively confined volumes; it can separately resolve multiplemarkers. It also displays low sensitivity to extraneous metal objects.

The magnetic tag or marker can typically be a 1 cm length (longer ifdesired) of amorphous wire (non-corrosive, diameter 90 micron or less)similar to that used in EAS tags or, with suitable process development,a short length (e.g. 1 cm) of a needle sputter-coated with a thin layerof soft magnetic material.

In use around the head of a patient, resolution to 0.1 mm with thedescribed markers can be achieved. Accuracy should also have thepotential to approach this value if some precautions about calibrationand use of other magnetic materials are observed, but for optimumperformance a rigid but open structure close to the head would bedesired. The magnetic field levels employed will be lower than thosegenerated by everyday magnets (e.g. kitchen door catches, etc.).

This technique has particular application to brain surgery, where thereis the requirement to locate the position of probes in three dimensionsand with high precision. It is therefore possible, in accordance withthis invention, to use small magnetic markers on such probes or needles.In this case, a key advantage is that the signal from the marker needonly be detected and resolved in time; the resolution is determined bythe location of the zero field plane, not by the signal-to-noise ratioof the detected marker signal. This permits a very small marker to beused.

A single axis position sensor may be implemented with a set of coilssimilar to the tag reading system described above. This comprises: apair of opposed coils carrying DC current to generate a DC fieldgradient; a means of applying a relatively uniform low level AC field todrive the marker in and out of saturation in the small region where theDC field is close to zero; and a means of applying a relatively uniformDC field of variable strength and polarity to move the location of theplane of zero DC field around the volume to be interrogated.

An anisotropic marker—i.e. one having a preferential axis ofmagnetization—resolves the magnetic field along its length. Such amarker can be obtained, for example, by using a long, thin element of amagnetic material or by suitable treatment of an area of magneticmaterial having a much lower aspect ratio, e.g. by longitudinallyannealing a generally rectangular patch of a spin-melt magneticmaterial. In the context of the single axis position sensor underdiscussion there are five degrees of freedom (x, y, z and two angles(rotation of the marker about its axis has no effect)). Three orthogonalcomplete sets of coils can capture sufficient information by doing threescans of the uniform DC field on each of the sets of coils in turn. Thefirst scan with no field from the other sets, the second with a uniformDC field from one of the other sets, and the third with DC field fromthe other set. This gives nine scans in all; these may be represented asin the following table, in which the magnetic field sources areidentified as a, b and c and the scans are numbered from 1-9 (scanningorder being of no significance)

Ortho- gonal field source 1 2 3 4 5 6 7 8 9 a ON ON ON OFF OFF ON OFFOFF ON b OFF ON OFF ON ON ON OFF ON OFF c OFF OFF ON OFF ON OFF ON ON ON

The only information required from each scan is the position of thecenter of the harmonic output from the marker within that scan. Thesenine DC field values can then be converted into the xyz-theta-phicoordinates of the marker. To start with, the system can simply be usedby holding the marker in the desired position before the head is putinto the coils; and then when the head is placed in the coils the markercan be moved until the same signals are obtained.

An alternative to sequential interrogation which has the advantage ofrequiring less time to scan the region of interest is to rotate themagnetic field gradient continuously so as to scan all directions ofinterest. This can be accomplished by driving three sets of coils withappropriate continuous waveforms. For example, a suitable scanning fieldwill be created if coils in the x, y and z planes are driven withcurrents I_(x), I_(y) and I_(z) given by the equations:

I_(x)=cos ω_(a)t(A cos ω_(b)t−sin ω_(b)t.sin ω_(c)t)−sin ω_(a)t.cosω_(c)t

I_(y)=sin ω_(a)t(A cos ω_(b)t−sin ω_(b)t.sin ω_(c)t)+cos ω_(a)t.cosω_(c)t

I_(z)=A sin ω_(b)t+cos ω_(b)t.sin ω_(c)t

where: ω_(a)=overall frequency of rotation of applied magnetic field

ω_(b)=null scanning frequency

ω_(c)=interrogation frequency

A=amplitude ratio ω_(b): ω_(c).

Typical (but non-limiting) values of these parameters are:

A=10;

frequency ratio ω_(a):ω_(b)≅1:10; and

frequency ratio ω_(b):ω_(c)≅1:400.

DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates the fundamental elements of a tag reading system ofthe invention;

FIG. 2 is a circuit diagram illustrating one mode of generating thedesired magnetic field pattern with the arrangement of FIG. 1;

FIG. 3 relates the magnetic response of a tag to its position within thereading system of FIG. 1;

FIG. 4 illustrates where magnetic nulls occur with a permanent magnet;

FIG. 5 illustrates an embodiment of the invention which utilizes a coiland a permanent magnet to generate the desired field pattern;

FIG. 6 illustrates an embodiment of the invention which utilizes a pairof permanent magnets to generate the desired field pattern;

FIG. 7 illustrates an embodiment of the invention which utilizes aplurality of permanent magnets disposed in an annular array with a coilto generate the desired field pattern;

FIG. 8 is a schematic circuit diagram for one embodiment of a taginterrogator in accordance with the invention;

FIG. 9 illustrates a selection of tags in accordance with thisinvention; and

FIG. 10 illustrates an embodiment of the invention as applied tosurgical operations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a schematic arrangement is shown in which a tag 1is positioned mid-way between two coils Tx1 and Tx2. The tag is of thetype shown in FIG. 9a, i.e. a simple linear tag carrying a plurality ofmagnetic elements each of which is a high-permeability magnetic alloymaterial, for example Vacuumschmeltze 6025 spin melt ribbon having anintrinsic permeability of about 10⁵. The reader will appreciate that thevalues given in this description for the various parameters associatedwith the elements shown in FIG. 1 are given merely by way of example,and illustrate one working embodiment. The values of these parameterswill inevitably vary according to the overall size of the system and itsintended function. The magnetic elements which constitute the discretemagnetically active regions of the tag have dimensions 10 mm×1 mm×25microns; the spacing between adjacent elements is 1 mm. The two coilsare spaced apart by approximately 20 cm and each comprise 450 turns of0.56 mm copper wire wound in a square configuration typically 45 cm×45cm. Each coil has a resistance of GD and an inductance of 100 mH. Eachof the coils Tx1 and Tx2 carries a direct current I superimposed uponwhich is a smaller alternating current i; typically, the direct currentI is of the order of 3A while the superimposed alternating current I isof the order of 50 mA. The alternating current i is of relatively highfrequency, typically about 2 kHz.

With a system such as that just described, the alternating and directcurrents in the two coils generate a magnetic field pattern in whichthere is a magnetic null in the direction of arrow x at points lying ina plane parallel to the two coils and mid-way between them. In FIG. 1,the x- and y-coordinates of this mid-way plane are represented by thelines 2 and 3, respectively.

If a magnetic tag of this invention is passed through the two coilsshown in FIG. 1, travelling in direction x and generally along thelongitudinal axis defined between the center points of the two coils, ftwill pass through a magnetic field polarity inversion at the mid-wayplane defined by coordinates 2 and 3. The change in polarity of themagnetic field comes about because the DC current flows in one sense inthe first of the coils and in the opposite sense in the other of thecoils, as indicated by the bold arrows in FIG. 1. At the mid-way plane,the magnetic field component generated by the direct current flowing inthe first coil exactly cancels the magnetic field component generated bythe direct current flowing in the other coil.

As the tag travels through the center of the first coil, it experiencesa high magnetic field which is sufficient to saturate its magneticallyactive elements; as the field strength decreases on moving towards themid-way plane, the magnetic material is influenced by the decreasingmagnetic field in a way dictated by its hysteresis curve. In thevicinity of the magnetic null, the direction of magnetization of themagnetic elements of the tag is reversed.

The relatively high frequency alternating current i shown in FIG. 1 isidentical in each of the coils Tx1 and Tx2.

The alternating current can have a frequency within a wide range, asindicated hereinbefore; a typical operating value with the arrangementof FIG. 1 is about 2 kHz. The effect of this relatively low amplitudealternating current is to cause the mid-way plane defined by coordinates2, 3 to oscillate about the geometric midpoint along the longitudinalaxis defined between the midpoints of the two coils. In other words, theplane containing the magnetic null oscillates or wobbles back and forthover a small spatial region at the frequency of the alternating current.

FIG. 2 shows a simple circuit for providing opposed DC fields combinedwith AC fields. Capacitor C1 is selected to resonate with the inductanceof coils Tx1 and Tx2 at the AC drive frequency; each of these coils hasa resistance of 6 ohms and an inductance of 100 millihenries. A typicalvalue for C1 is 0.1 μF. C2 is a capacitor selected to behave as aneffective short-circuit at the AC drive frequency; a typical value forthis component is 22 μF. The DC power supply will typically provide 30volts at 3 amps; and the AC source will typically deliver an alternatingcurrent at a frequency of 2 kHz at 2 v rms.

FIG. 3 illustrates how the magnetization of a single magnetic elementvaries with time at different positions within the magnetic fieldpattern defined between the coils Tx1 and Tx2 of FIG. 1. For ease ofillustration, the oscillation of the plane containing the magnetic nullis represented by the bold double-headed arrow (⇄) 4, the extremepositions of the plane being represented by dashed lines 5 and 6,respectively, and the mid-point between limiting planes 5 and 6 beingrepresented by dashed line 7. In the right hand portion of FIG. 3, theapplied AC field is shown varying with time between positive (H+) andnegative (H−) field values. Beneath the graph of the applied AC field,there are five graphs depicting how the net magnetization of themagnetic element varies with time in each of five geometric positionsindicated to the left as Position 1, Position 2, etc. Planes 5 and 6define the limits of regions within which magnetic field polarityreversals occur. In practice, the separation between planes 5 and 6 istypically of the order of 1 mm; for a given magnetic material, thisdistance can be increased or decreased at will within certain limits byvarying the amplitude of the AC current and/or the DC current in thecoils.

At all times, the magnetic element has a linear magnetic axis which isorthogonal to the planes 5, 6 and 7.

In Position 1, the end of the magnetic element is adjacent to plane 6;in this condition, it experiences a positive magnetic field at all timesand its net magnetization is time-invariant. In Position 2, the leadingend of the element has reached the mid-way plane 7. Most of the magneticmaterial, however, still remains outside limiting plane 6. Inconsequence, the null plane is able to interact with only a portion ofthe magnetic material, resulting in a time-variable net magnetizationhaving the repeat pattern shown, i.e. a straight line positive-valueportion followed by a generally sinusoidal arc which dips towards zeroand then rises to its original positive value.

In Position 3, the magnetic material is positioned symmetrically withrespect to the mid-way plane 7. Here, the net magnetization versus timeplot consists of a sine wave whose frequency corresponds to that of theapplied AC field. In Position 4, the majority of the magnetic elementexperiences a negative field at all times, while a smaller part of theelement experiences polarity reversals; this leads to the netmagnetization versus time plot as shown. The fact that Position 4 is ineffect the inverse of Position 2 is reflected in the relationshipbetween the magnetization plots for these two positions; as can be seen,the plot for Position 4 is effectively a mirror image of that forPosition 2 but with the curved portions time-shifted.

Finally, that Position 5, all of the tag experiences the negative field,and no part of the tag experiences field polarity reversal. Inconsequence, the net magnetization is time-invariant, being a constantnegative value as shown.

When a tag containing such a magnetic element is passed along the coils'axis through the region of zero field, it will initially be completelysaturated by the DC magnetic field. It will next briefly be driven overits B-H loop as it passes through the zero field region. Finally it willbecome saturated again. The portion of the traverse over which themagnetic material is “active”, i.e. is undergoing magnetic changes, isphysically small, and is determined by the amplitude of the DC field,the amplitude of the AC field, and the characteristics of the magneticmaterial. This region can easily be less than 1 mm in extent. If thelevel of the alternating field is well below that required to saturatethe magnetic material in the tag, then harmonics of the AC signal willbe generated by the tag as it enters the zero field region (Positions 1to 2) and responds to the changing field. As the tag straddles thenarrow zero field region (Position 3) the tag will be driven on thelinear part of its B-H loop, and will interact by re-radiating only thefundamental interrogation frequency. Then, as the tag leaves the zerofield region, (Positions 4 to 5) it will again emit harmonics of theinterrogation field frequency.

A receiver (Rx) coil arranged to be sensitive to fields produced at thezero field region, but which does not couple directly to theinterrogator (Tx) coils, will receive only these signals. Such anarrangement can be achieved by using separate Tx and Rx coils physicallyarranged to have low mutual coupling; or by using a single coil (havingboth Tx and Rx functions) together with suitable filtering in the Tx andRx paths. The variation of these signals with time as the tag passesalong the coils' axis gives a clear indication of the passage of theends of the magnetic material through the zero field region.

The result of this interaction between the tag and the magnetic field itexperiences is shown in FIG. 3b. Here, the region 4 over which themagnetic null oscillates is shown on a smaller scale, and the numbereddots represent the location of the mid-point of the tag in each ofPositions 1-5. The generation of a harmonic signal by the tag(illustrated by the second harmonic of the applied frequency) isapparent at positions where the tag enters the region defined bylimiting planes 5 and 6, i.e. the zone where magnetic field polarityreversals occur. Because of the symmetry of the system, a singlemagnetic element will generate a doublet peak 8 a and 8 b sincePositions 2 and 4 are redundant.

Referring now to FIG. 4, this illustrates the lines of force (i.e. themagnetic contours) existing with a simple bar magnet. The plane X-Ywhich intersects the longitudinal axis of the bar magnet and which isorthogonal to the plane of the paper constitutes a magnetic null plane.Thus a magnetic element possessing a sensitive magnetic axis alignedorthogonally with respect to the null plane will experience a magneticnull as it traverses either path A-B or path C-D. Consequently a simplebar magnet can be used as part of an interrogation system to detect thepresence of such a magnetic tag, or to read information carried by sucha tag.

The generation of second harmonic signal can form the basis of a tagdetection system. If, instead of just a single magnetic element the tagincludes a linear array of n magnetic elements, the second harmonicoutput from the tag will comprise n duplet peaks, each of the type shownin FIG. 3b. If the size and magnetic characteristics of the magneticelements are all the same, the peaks will have the same profile and eachpeak will define an envelope of constant area. The spacing betweenindividual magnetic elements will influence the relative positions ofthe duplet peaks on an amplitude versus time plot. It will beappreciated that the present invention is not restricted to the use ofsuch simple tags as just described. The use of magnetic elements ofdifferent sizes and magnetic characteristics, and with non-uniformspacing along the length of the magnetic tag, will generate more complexsignal patterns which nevertheless are characteristic of the given tagconstruction. By varying the number, the magnetic characteristics, andthe positioning of a series of magnetic elements, it is possible tomanufacture a very large number of magnetic tags each with its ownunique characteristics which will accordingly generate a unique signalwhen used in conjunction with the system of FIGS. 1-3.

It will also be appreciated that the invention is not limited toobserving the second harmonic of the applied alternating frequency; thisparticular harmonic has been selected for the purposes of illustrationsince it is relatively easy to generate a transmit signal (Tx output)which has no (or very little) second harmonic content, thus permittinggood discrimination between the Tx signal and the response of the tag;and since it also contains a relatively high proportion of the totalharmonic energy output from the tag.

Referring next to FIG. 5, there is shown a schematic arrangement for asimple tag reader in accordance with this invention, the readerutilizing a permanent magnet 10 and a coil 11 located adjacent to oneface of the magnet. In this embodiment, a tag which is to be read can bepassed along path C-D through coil 11 or along path A-B above the coil.The tags must be oriented with their magnetic axis aligned with thedirection of tag movement. In FIG. 5, the magnetic null plane ispositioned at 12 as shown.

Referring next to FIG. 6, the use of two permanent magnets positionedwith their magnetic axes aligned and with like poles opposing oneanother is illustrated. Such an arrangement generates a null plane 13;the direction of tag motion required is indicated by arrows 14. Again,the magnetic axis of the tag must be aligned with the direction ofmovement.

FIG. 7 shows a simple realization of a tag reader head using a pluralityof permanent magnets to generate a magnetic null plane. As illustratedten polymer-bonded ferrite magnets are disposed in an annular array withlike poles facing inwards. A common transmit/receive coil L1 sits withinthe annulus of magnets in the manner indicated. The tag is read as itpasses through the null plane in the center of the loop of magnets.

Referring next to FIG. 8, there is shown one embodiment of aninterrogation system in accordance with this invention. This is based onthe use of a single coil L1 to act as both transmitter (Tx) coil, whichgenerates the desired magnetic field pattern, and as the receiver (Rx)coil. The system uses the second harmonic output of the tag as the oasisfor tag detection/identification. Circuit components C1 and L2 form aresonant trap at frequency 2f to reduce signals at this frequency in theTx output to a very low level; C2 resonates with L1 at frequency f; andcomponents C3, C4, L1 and L3 form a filter to pass wanted signals fromthe tag at frequency 2f while rejecting signals at the transmittedfrequency f.

The output obtained from this circuit passes through a low pass filterto an analogue to digital converter (ADC) and thence to a digital signalprocessor. These components, and in particular the signal processor,will be configured to suit the intended application of the interrogationunit. The nature of the signal processing, and the means by which it isachieved, are all conventional and therefore will not be describedfurther here.

FIG. 9 illustrates the basic structure of magnetic tags in accordancewith the invention. FIG. 9a shows a tag 100 which comprises a supportmedium 101 (e.g. paper or a plastics material) and a linear array ofmagnetically active regions 102, 103, 104, 105 and 106. Eachmagnetically active region is formed from a patch of high-permeabilitymagnetic material (e.g. Vacuumschmeltze 6025) having its magnetic axisaligned along the length of the tag. Each patch is about 10 mm² in areaand is adhesively secured to the substrate 101.

Patches 102-105 are identical in dimensions and magnetic properties, andare uniformly spaced apart, gaps 110, 111 and 112 all being the same.The gap between patches 105 and 106, however, is larger—as though therewere one patch missing at the position indicated by dotted lines at 113.

Tag 100 behaves as a six-bit tag, coded 111101 (the zero being area113).

A functionally equivalent tag 120 is formed of a substrate 121 carryingmagnetic elements 122-126 and having a “gap” 127; in this embodiment,the magnetic elements are in the form of a strip or wire ofhigh-permeability magnetic material (e.g. Vacuumschmeltze 6025),typically being about 5 mm long, 1 mm wide and about 15 microns inthickness.

FIG. 9b illustrates an alternative construction for a six-bit, laminatedtag 130. This tag is coded 111101, as in FIG. 9a. Here, a continuouslayer or length of high permeability magnetic material 131 (in the formof wire, strip, thin film or foil) and a substrate 133 have sandwichedbetween them a magnetic bias layer 132. The bias layer is magnetized inpredetermined areas which influence the overlying high permeabilitymaterial to generate magnetically active regions indicated as 134, 135,136, 137 and 138. Region 139 is not active, and thus constitutes amagnetic zero. When read by an interrogation system such as that of FIG.8, the output generated by tags 100, 120 and 130 will be as shown inFIG. 9d.

A more complex tag is shown in FIG. 9c. Here there are a series ofparallel linear arrays of magnetically active material, generating a 4×4array of sites where the magnetically active material may be present(coding as ‘1’) or absent (coding as ‘0’).

FIG. 10 illustrates the general arrangement of three sets of coils asused in accordance with this invention for surgical applications. Thethree sets of coils are all mutually orthogonal and define a cavity intowhich the head 200 of a patient may be positioned. The first coil setconsists of coils 201 a and 201 b; the second set consists of coils 202a and 202 b; and the third set consists of coils 203 a and 203 b. In thedrawing, two surgical probes 204 and 205 are shown schematically inposition within the patient's cranium. The probes each have, at theirdistal ends, a magnetic tag 206, 207 such as one of those described withreference to FIG. 9 above. Because the magnetic element of the tag isonly required to provide information of its presence (rather than holdextensive data), relatively simple tags are preferred. A single magneticelement of high permeability magnetic material located at the tip of theprobe is sufficient. The coils are operated in the manner described indetail hereinabove. By means of the present invention, it is possible todetermine the positions of the ends of the probes with highprecision—and thus to carry out delicate surgical procedures withaccuracy and with minimum damage to healthy tissue.

What is claimed is:
 1. A magnetic marker or tag which is characterizedby a substrate carrying a plurality of discrete magnetically activeregions disposed in one or more linear arrays, the magnetically activeregions being formed of a thin film or spin-melt material having apreferential axis of magnetization, and the preferential axes ofmagnetization being aligned within the (or each) linear array.
 2. A tagas claimed in claim 1, characterized in that either the magneticallyactive regions, or the spaces between said magnetically active regions,or both, is or are non-uniform.
 3. A tag as claimed in claim 1,characterized in that said discrete magnetically active regions areformed from a continuous area of magnetisable material discrete regionsof which are magnetized so as to generate space(s) between saidmagnetically active regions.
 4. A tag as claimed in claim 1,characterized in that the tag has two linear arrays disposedorthogonally.
 5. A tag as claimed in any one of claims 1, 2, 3 or 4,characterized in that it is in the form of a relatively long, thin striphaving a preferential axis of magnetization along its length.
 6. A tagas claimed in claim 1 or 2, characterized in that the preferential axesof magnetization are generated by longitudinal annealing of a spin-meltmaterial.
 7. A tag as claimed in claim 2, characterized in that thespacing between each of the magnetically active regions is non-uniform.8. A tag as claimed in claim 1, characterized in that each of themagnetically active regions is of substantially the same shape and size.9. A tag as claimed in claim 1, characterized in that the spacingbetween each of the magnetically active regions is uniform.